It is definitely a stone. I too would think it would have made a racket. I wonder if it happened while my son was driving it and he neglected (was afraid) to tell me. I can't figure out how it got in there. The stone is roughed up pretty good from the impeller. The impeller now won't turn. That stone has got to be 3/4 inch in size. I think someone put it there...

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Muleears
'07 E320 Bluetec 133K my DD
'04 Jaguar XJ8 VDP, 34K
'10 Hyundai Accent 60K Grocery Getter
'02 VW Golf soon to be on the road again
'97 E300 Diesel Son's DD
'61 VERY tolerant wife

Hampton Roads, VA USA

Gone but not forgotten:
'67 250S 95K
'86 300SDL
'87 300D Turbo, 364K! R.I.P.
'98 E300 Turbodiesel, 213K
'02 S420, 164K
'01 Prius 138K